Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

Abstract

WO₃ is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO₃ valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO₃ surface can improve the kinetics for water oxidation and increase the branching ratio for O₂ production. Ir-based OECs were attached to WO₃ photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO₃ photoanodes in 1 M H₂SO₄. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO₃/electrolyte interface. Sputtering of IrO₂ layers on WO₃ did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O₂ yield at 1.2 V vs. RHE from ∼0% for bare WO₃ to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO₂/WO₃ junction, which provided a shunt pathway for electrocatalytic IrO₂ behavior with the WO₃ photoanode under reverse bias. Although other OECs were tested, only IrO₂ displayed extended stability under the anodic operating conditions in acid as determined by XPS.